We’re a week or so beyond Yellowstone, but I just wanted to say a few words on geysers, since
G covered the beasties.

There are only a couple of places in the world where you can find geysers: Yellowstone Park is home to more than half of them (other places are Chile, Russia, New Zealand and Iceland), which was a surprise. I only knew of Old Faithful and that
other one that has been polluted by the coins (and although pollution is of course really bad we all secretly think it is improved).
Apparently when tales of the geysers were first told back in the East they were disbelieved and it was the presence of the geysers not the wildlife which led to Yellowstone becoming the first national park.
Beneath Yellowstone lurks the Yellowstone Caldera which is a cauldron caused by the collapse of a super-volcano and which is the source of all the bubbles and eruptions. I’m told an eruption is not likely (maybe in 100,000 years) but when it does come it will be near apocalyptic for life within dead far and will have a significant effect on the world climate. Yellowstone is the biggest supervolcano in the world and when it last erupted 2 million years ago it was about 10,000 times as big as the 1980 St Helens eruption. Beneath the park is a giant pulsating chamber of magma which is 6 miles deep and 55 miles by 20 miles. There are also about 1000 to 3000 earthquakes every year (Now re-read this paragraph imagining Morgan Freeman narrating and try to restrain yourself from rushing out to Tesco express and stocking up on bottled water and tins of sardines).
The history of Yellowstone is a history of learning how to manage a National Park and –more importantly- how the idiocy and selfishness of the public cannot be underestimated. Old Faithful, for example, used to have a cone on the top but visitors broke bits off to take home and now it is gone. One of the geysers has stopped working because people threw their handkerchiefs into it to watch them erupt out, and then they threw rocks in and now it is blocked up with frickin rocks. People have also climbed into the geysers for a warm sit-sit, brain not thinking about the source of the hots until the split second before they got fried to a crisp (like in one of Pierce Brosnan’s finest vehicles: Dante’s inferno).
I wish I could say that we have moved beyond all of that but the advent of smart phones and selfie sticks has brought a whole new shade of stupid. I witnessed a guy trying to take a closer picture of the geyser by pushing the phone over it on the end of a selfie stick- not sure he really wanted a $700 iFried but obviously the brain was on vacation too.
We stayed at the Old Faithful Inn which is home to Old Faithful who erupts every 90 minutes (give or take about 10 minutes).

So where are these 400+ geysers? Everywhere! Lots of them are in areas that you can walk through for a few miles and see several but others are just happily bubbling away at the side of the road. Pretty they are but it was, on occasion, like being on the edge of the bog of eternal stench.
Hot springs create different water temperature environments. As the water gradually cools it creates lower temperature environments ideal for different species of algae and bacteria- the different colours represent different species each requiring a different temperature environment
Geysers are hot springs that periodically erupt. They are then grouped into two categories cone geysers (narrow openings/ vents which act like a nozzle spouting water) and fountain geysers (water collects in a wide opening/vent before and during eruption)
The hottest of Yellowstone’s geothermal features are fumaroles (steam vents).They rapidly boil away what little water they contain and release steam and other gases forcefully (and often noisily) from underground.
And finally not forgetting the mudpots:
These make exactly the idiotically amusing noises that you would expect.